This invention relates in general to gain control circuits and specifically to a novel gain control circuit that uses a pulse width modulator (PWM) as the gain control element. There is a need in many electronic applications to control the gain of an amplifier by means of a DC control voltage. Some of these applications include audio compression-expansion arrangements, AGC circuits, noise reduction circuits, digital controls for audio signals such as volume and tone, television raster correction circuits, analog multipliers and active noise cancellation circuits. It is well known to use analog multipliers to perform gain control functions. The invention provides a novel gain control that changes the transfer function characteristics of a PWM for effecting a gain control action.
In the particular application described, namely the control of raster correction signals in a television environment, the feature of using the frequency of the ramp voltage of the PWM for gain control is especially desirable. Such an arrangement will find ready application in television receivers and monitors that are designed to operate at several different scanning frequencies and in different modes. In such a multi frequency television monitor application, the voltage supply to the television scanning circuitry must be increased as the scanning frequency is increased while the voltage supply is modulated by a correction waveform to produce well known raster correction for distortions such as pin cushion, keystone, etc. on the cathode ray tube. For proper tracking of the raster correction, the amplitude of the raster correction signal must be held as a constant percentage of the supply voltage. Consequently, as the scanning frequency increases, the required amplitude of the correction signal increases also. The specific television deflection system application of the present invention is especially well suited for this purpose.
The pulse width modulator is formed by a voltage ramp generator, and a comparator or very high gain amplifier. Its output is coupled to an integrator. A low pass filter is also useful to filter the signal at the output of the integrator. As illustrated in FIG. 1, the transfer function characteristics of a pulse width modulator, formed by a ramp generator and a comparator, are altered to change the gain of the circuit. Consequently, the amplitude of the output waveform can thus be modulated.
Gain can be determined by the following formula: EQU Gain=V.sub.cc .div.V.sub.rmp.
Thus the output voltage may be determined by V.sub.out is equal to V.sub.cc .div.V.sub.rmp .times.V.sub.in where,
V.sub.rmp is the peak to peak ramp voltage at the pulse width modulator output; PA1 V.sub.cc is the supply voltage to the output for the PWM; PA1 V.sub.in is equal to the input signal at the comparator input; and PA1 V.sub.out is equal to the output voltage after integration.
As shown by the above formula, changing the gain of the circuit may be accomplished by changing either V.sub.cc or V.sub.rmp. Therefore, the output signal amplitude can be made to be a gain altered replica of the input signal by controlling one or more of the transfer function characteristics of the PWM.